Methods and devices for determining the composition of fuel mixtures are believed to be understood from the related art. For example, the use of fuel mixtures which, in addition to the actual mineral oil fuels, are able to process an admixture of ethanol and/or other alcohols is increasing in the motor vehicle sector. As a rule, the parameters of the engine control of the motor vehicle are adapted to the composition of the fuel mixture. However, in some cases this may require knowledge of the composition of the fuel mixture, especially an ethanol/fuel mixture ratio. One measuring method consists of measuring an absorption, transmission or reflection of microwaves through the fuel mixture. An example of a device set up for this purpose is discussed in DE 34 12 704 A1.
Moreover, a method for determining a composition of a fuel mixture is discussed in post-published DE 10 2008 044 383.2. Here, a characteristic behavior of a particular response to incident microwave radiation is recorded across a greater frequency range in order to determine the characteristics of the fuel mixture, and the composition of the fuel mixture is inferred therefrom. One particular advantage of the method discussed in DE 10 2008 044 383.2 is that it can increase the precision by using greater frequency ranges, so that, for example, alcohol-fuel mixtures that additionally also contain a water component and/or additives, are able to be characterized much more accurately than when using conventional methods. However, there is potential for improvement with regard to the incoupling and outcoupling of the microwave signals into or out of the fuel.